Severe acute respiratory syndrome (SARS), caused by a novel coronavirus known as SARS-CoV, is a highly contagious and life-threatening respiratory disease, with the lungs as its main pathological target. The exact mechanism of SARS pathogenesis remains unknown. In SARS patients, extensive epithelial disruption is seen with typical coronavirus-like particles in the cytoplasm of pneumocytes, indicating that epithelial cells are not only permissive to SARS-CoV but also important in the pathobiology of SARS. Importantly, we have shown that human bronchial epithelial Calu-3 cells responded to SARS-CoV infection by producing several NFkappa B-mediated cytokines, including IL-6 and IL-8. Because SARS is considered a viral pneumonia, likely caused by unregulated and often excess innate inflammatory responses, a full understanding is urgently needed as to how these inflammatory factors secreted by SARS-CoV-infected lung epithelial cells would interact with macrophages and dendritic cells (DC), two of the key cellular elements of the innate immune system, with regard to the induction of a cascade of inflammatory responses that ultimately influence the outcome of infection. Based on the strong preliminary data generated with this highly pathologically relevant Calu-3 cell model of SARS-CoV infection, we hypothesize in this application that cytokines released from highly differentiated Calu-3 cells that were infected by SARS-CoV will alter intrinsic functions of both macrophages and DC, resulting in exacerbated inflammatory responses. We also hypothesize that attenuation of the initial inflammatory secretion by SARS-CoV-infected lung epithelial cells will alleviate the subsequent macrophage and/or DC-mediated inflammatory responses. Two specific aims will be pursued: (1) Define the role(s) of human lung epithelial cells in the host responses to SARS-CoV. The working hypothesis, based on preliminary data, is that innate inflammatory mediators secreted by SARS-CoV-infected human lung epithelial cells are potent modulators of macrophages and DC, leading to cascades of systemic inflammatory responses (a cytokine storm?). (2) Identify SARS-CoV-induced innate signaling pathways in human lung epithelial cells. The hypothesis to be tested is that SARS-CoV activates Calu-3 cells through TLR-3-, TLR-8, and/or RIG-I-mediated pathways, resulting in secretion of NFkappa B-dependent inflammatory factors. Upon completion, these studies will provide insight into SARS pathogenesis and cellular targets for therapeutic and preventive measures against SARS. SARS is a highly contagious and often fatal human respiratory disease. The pathogenesis of SARS remains unknown and effective therapeutic measurements are not available. This project is of relevance to public health, because it focuses not only on determining the cellular and molecular basis for SARS pathogenesis, but it also on identifying key inflammatory "biomarkers" that will be useful for diagnostic and therapeutic interventions against future outbreaks of SARS.